This is to show that you can animate using Paint Tool Sai or Gimp or other Layered software without the need of buying expensive softwares unless you have the money to buy it. Animation on Paint Tool Sai takes hell a lot of patience and effort. It's not easy to animate and don't think animation will be a fly since animation takes time.

If you can afford softwares such as Toon Boom, it will be best as Toon Boom is way faster than Animation on Paint Tool Sai or any software and also animation friend. Toon boom is rivaled with Retas and Retas is famous in Japan. The show such as Naruto, Bleach, Shakugan no Shana, Haruhi Suzumiya and more are all animated using Retas. TV Paint is famous in Europe and Toon Boom is a Canadian software famous in Disney Animated.

Yep, I've used Flash on my Animation school before but I stopped using it when I got ToonBoom. Toonboom is completely identical to Flash only that has more power, 3D capabilities, lots of inverse and reverse kinematics. Soothing for frame by frame and one of the most expensive feature is it's vector due to resolution independent so I have no worries what quality the project is made, it will go high quality as long as quality data is saved.